Helicopter rotor designs are increasingly utilizing a flexible structural member, commonly termed a "flexbeam" or "flexbeam connector", for retention of a helicopter rotor blade to a torque drive hub member. Basic operational constraints of rotary wing flight impose substantial functional complexity upon the rotor flexbeam necessitated by various needs to control accurately multi-directional displacement of the rotor blades, i.e., flapwise and edgewise bending, and torsional or pitch change motions. As such, these configurations are termed "Bearingless Rotors" inasmuch as they replace antiquated bearing element rotors which accommodate motion by hinge or journal type bearings at the rotor blade root end. The flexbeam connector, which is typically comprised of fiber reinforced resin matrix materials, reduces the weight, complexity, and maintenance of the rotor assembly while, furthermore, improving the reliability and damage tolerance thereof.
Bearingless rotors of the varieties described in U.S. Pat. Nos. 4,244,677, and 5,092,738 typically include a torque tube member enveloping each of the flexbeam connectors for imparting pitch motion to the rotor blades. The torque tube member rigidly mounts outboard to the root end of the rotor blade and articulately mounts inboard to the upper and lower surfaces of the flexbeam connector. The articulate mount is effected by a centering bearing, commonly identified by the appellation "snubber bearing", which performs the functions of centering the torque tube member relative to the flexbeam connector for pitch change and flapping motion, accommodating lead-lag motion between the torque tube member and the flexbeam connector and transferring pitch control and other loads therebetween. Centering bearings, such as those described in the above-identified U.S. Patents, are typically comprised of a plurality of spherical and flat elastomeric laminates, which spherical laminates accommodate pitch change and flapwise bending motion and which flat laminates permit a small degree of radial and a larger degree of edgewise motion. The flat laminates are, furthermore, comprised of high loss elastomer material for providing edgewise, or lead-lag, vibration damping.
Pitch control inputs are imparted to the rotor blade assembly i.e., the torque tube, flexbeam connector and rotor blade, by pitch control rods which are articulately mounted at an upper end to the torque tube and pivotally mounted at an opposing end to a swashplate assembly. The spatial displacement of the swashplate causes linear displacement of the pitch control rods to effect rotational displacement of the torque tube member about the pitch axes of the respective rotor blade. The torque tube member, therefore, imparts pitch motion to the rotor blade to vary the flight profile, e.g., speed, pitch, roll of the helicopter.
In addition to the primary function of providing pitch control inputs to the rotor blades, the pitch control rods of the prior art provide tracking correction for the corresponding rotor blade. Manufacturing deviations and rotor blade erosion can vary the aerodynamic characteristics of a rotor blade, which deviations can cause the rotor blades to track dissimilar tip-plane paths when in operation. Such "out-of-track" rotor blade condition results in increased rotor vibration and degraded aerodynamic performance. Pitch change adjustments for correcting rotor blade tracking errors are typically made by moving the swashplate to a static reference position and adjusting the axial length of the control rod to alter the initial pitch setting i.e., angle of attack, of the rotor blade. Generally, only small adjustments are required, on the order of .+-.1 degree, to effect the necessary corrections to properly track the rotor blades.
FIG. 1 depicts a prior art pitch control rod 200 comprising upper and lower rod end portions 202, 204, each having threaded shaft end portions 206, 208 which are mechanically interconnected by a threaded barrel member 210. The barrel member 210 functions similar to the operation of a turnbuckle insofar as rotation of the barrel 210 causes simultaneous axial displacement of the upper and lower rod end portions 202, 204. Axial displacement is effected by the use of right and left handed threads on the shaft end portions 206, 208. The barrel 210 is locked in position by upper and lower jam nuts 212, 214 and redundantly retained by a locking key 216 which engages one of a plurality of circumferential castellations 218 formed along the upper end of the barrel 210. The locking key 216, which also engages an axial slot 220 formed in the upper threaded shaft end portion 206, is retained in one of the circumferential castellations 218 by the upper jam nut 212 thus providing a positive anti-rotational feature for the pitch control rod 200. Accordingly, by repositioning the barrel 210 i.e., turning the barrel clockwise or counterclockwise, the length of the pitch control rod is varied to effect pitch position adjustments of the torque tube member and, consequently, the rotor blade assembly.
While the prior art pitch control rod 200 described hereinabove is functionally adequate for providing pitch control inputs to the rotor blade assembly and for providing tracking correction therefor, the pitch control rod has certain inherent disadvantages and limitations. For example, the threaded and slotted components, i.e., the upper and lower rod end portions 202, 204 of the barrel 210, and the upper and lower jam nuts 212, 214, are not amenable to lightweight composite material construction. As such, the control rods of the prior art are typically fabricated from a metallic material such as steel or titanium, which increases the overall aircraft weight. The pitch control rod is also highly mechanically complex which results in increased fabrication and maintenance costs, and furthermore requires additional procedural steps to ensure that all safety requirements, i.e., proper torque loading of the jam nuts and engagement of the locking key etc., have been attended to. Moreover, the multiplicity of component parts increases the number of possible failure modes and the probability of operator error.
A need, therefore, exists to provide a pitch adjustment assembly for rotor blade tracking which is lightweight, simple to use, and requires fewer component parts than existing pitch adjustment mechanisms.